Integrated services digital network (ISDN) communication systems enable telephone service providers to supply multiple types of signalling channels from a central office over a signal local loop twisted pair to a network termination interface at a customer premises site. FIG. 1 diagrammatically illustrates a reduced complexity example of a typical extended distance ISDN communication system, while FIG. 2 diagrammatically illustrates a non-extended distance ISDN communication system. The extended distance ISDN system of FIG. 1 is intended to enable a telephone service provider to supply multiple types of signalling channels from a central office (CO) site 10 over a PCM link (such as an optical fiber link) to a remotely located customer premises equipment (CPE) site 20, while the non-extended network of FIG. 2 provides a local loop directly from the central office to the customer premises site. ISDN signalling channels typically include digital data channels and/or digitized voice channels, as well as a separate administrative channel that conveys call control information.
More particularly, in the extended distance system of FIG. 1, the central office site 10, which is located at a `west` end of an extended (PCM) communication link 30, includes a central office switch 11 (such as a 5ESS switch manufactured by AT&T), which contains a plurality of line termination circuits (or line cards), one of which is shown at 12. For extended distance communications carried out by the system of FIG. 1, line card 12 is coupled over a local loop (twisted tip/ring pair) 13 to what is termed in the telephone industry as a U Basic Rate-One Transmission Extension (U-BR1TE) transceiver card 14 (also known in the industry as a Basic Rate Interface Transmission Extension (U-BRITE) card).
The U-BR1TE (or U-BRITE) card contains an ISDN transceiver unit and an associated PCM bus interface which are operative to transmit and receive standard 2B+D ISDN data traffic over a PCM digital data link, such as a T1=1.544 MB/s link, and to interface the ISDN signals via the local loop to and from loop termination equipment. Namely, the U-BR1TE card 14 interfaces the local loop 13 with a digital data link 30 for PCM communications transport to another U-BR1TE transceiver card 24 at the east end of the link 30, serving the customer premises equipment (CPE) site 20. The U-BR1TE transceiver card 24 is, in turn, coupled over a local loop (twisted tip/ring pair) 23 to a network termination interface (NT-1) circuit 21, to which customer premises equipment, such as an ISDN data terminal, shown at 22, is coupled. In the non-extended system configuration of FIG. 2, the local loop 13 from a respective line card 12 in the central office switch 11, rather than being coupled to a U-BR1TE transceiver card, is coupled directly to the network termination interface (NT-1) circuit 21.
In either system configuration, since the network termination interface 21 is customer-installed (and therefore locally powered at the customer premises site 20, rather than being powered by the central office 10, as in the case of a conventional POTS telephone), the local telephone service provider is unable to interfere with the customer's choice of equipment to be connected to the ISDN line, such as the installation of equipment that provides a battery back-up in the event of a power outage.
Although the public service commissions of local jurisdictions where customer premises equipment is installed require that telephone service providers (regional Bell operating companies) ensure that each customer be provided with emergency (911) telephone service, the service provider is currently not permitted to dictate that the customer install a particular piece of equipment, such as one that will provide back-up battery for the network termination interface during a power outage. As a consequence, service providers customarily require their residential customers to maintain a separate POTS line as an emergency adjunct to the ISDN service, so that the residential customer is, in effect, forced to subscribe to an additional form of `fire insurance`. Namely, the extra POTS line is intended to prevent residential customers from losing access to telephone service, particularly in the event of an emergency, where expedient 911 service may be critical. Such a requirement obviously constitutes a substantial cost penalty to the customer, who is faced with both a potential installation fee, and an unwanted monthly bill for a normally unused POTS line.